test/suites: Unset join_token_expiry when done
test/suites: Unset join_token_expiry when done

This unsets the join_token_expiry config key after the test has
finished. This will make LXD use the default expiry of 3h.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd